Conventionally, a capacitive occupant detection system is known in the art. One system includes a mat shaped capacitive sensor and an electronic control unit (ECU) for detecting occupant. The capacitive sensor is responsive to differences of a weak electric field which is generated between a main electrode arranged inside a seat and a body member of the vehicle, and is arranged to output the differences in a current form or a voltage form. One example is disclosed in JP H11-271463A.
For example, if a seat is vacancy, only air is supplied between the electrodes of the capacitive sensor. In another situation, if a child restraint system (CRS), such as a junior restraint device, is mounted on the seat, the CRS or the CRS with a child is interposed between the electrodes. Further, if an adult human is on the seat, a large human body is interposed between the electrodes.
Here, the relative dielectric constant of air is about 1.0. The relative dielectric constant of the CRS is in a range from about 2.0 to about 5.0. The value varies depending on material of the CRS. The relative dielectric constant of a human body is about 50.0. Thus, the relative dielectric constants of air, the CRS, and a human body differ, respectively. Therefore, the capacitance between the electrodes of the capacitive sensor is changed according to the interposed objects.
The capacitive sensor is responsive to differences of a weak electric field caused by the difference of the capacitance. The capacitive sensor outputs the differences in a current form or a voltage form. Then, the ECU performs an occupant discrimination based on the output. That is, the ECU determines whether the seat is vacancy, the seat is mounted with a CRS, or the seat is occupied with an adult. In addition, an airbag ECU determines whether a bag member can be enabled to deploy or be disabled to deploy based on the determination result of the occupant detection ECU. In detail, the bag member is disabled to deploy, when the seat is vacancy or the seat is occupied with a CRS. On the other hand, the bag member is enabled to deploy, when an adult sits down on the seat.
It is known in the art that a capacitive sensor which detects water poured on the seat, and more clearly discriminates a case where the seat is occupied with a passenger and a case where the seat is vacancy. One example is disclosed in U.S. Pat. No. 7,497,465 (JP 2006-27591A).
If water is poured on the seat, it makes difficult to discriminate objects on the seat, since the relative dielectric constant of water is about 80.0 which is larger than the relative dielectric constant of a human body. For this reason, the above example of the capacitive sensor is newly installed with a sub electrode for detecting water. The sensor outputs differences of the weak electric field between the sub-electrode and the main electrode arranged inside the seat for occupant detection in a current form or a voltage form. The sensor enables the ECU to detect water on the seat.
The conventional capacitive sensor having the above-mentioned configuration outputs differences of the weak electric field generated between the electrodes in a current form or a voltage form. That is, different levels of current or voltage is generated according to conditions of the electric field between the electrodes, which reflect situations on the seat, such as whether a passenger sits down on the seat or not, what kind of passenger is on the seat, whether water is on the seat or not. The discrimination between a passenger and water is performed based on a current value or a voltage value. The current value and the voltage value may be hereinafter also referred to as a current value etc. or a detected value.
The current value etc. is outputted as a value which relates to both a capacitive component between the electrodes and a resistive component on a circuit providing the capacitive sensor. That is, when detecting the current value etc. between the electrodes in the capacitive sensor, the current value etc. is detected as a value affected by the influence of the resistive component of the circuit.
The resistive component includes resistances of objects, such as a human (passenger), water, and air etc., which is interposed between the electrodes. This is understandable, since a human body and water etc. can be expressed in an electric equivalent circuit which is a parallel circuit of a resistance and a capacitor.
Therefore, in a case of using a capacitive sensor, if occupant etc. is determined based on an amount of detected current value flowing between the electrodes, the detected current value includes current component flowing through the parallel circuit of the capacitor and the resistance which are formed between the electrodes. The occupant discrimination using the above current value as it is as a discrimination factor has an improvement limit in accuracy. That is, the occupant discrimination using the current value etc. does not use a pure capacitance between the electrodes as a discrimination factor, e.g., when water is poured on the seat. Therefore, there may be a case where it is impossible to discriminate exactly.
There is a conventional technology which can improve discrimination accuracy among a passenger and water etc. For example, a capacitive occupant detection sensor is disclosed in US Application Publication 2008/0100425 (JP 2008-111809.) The sensor includes a power supply section which generates alternating voltage, a main electrode arranged in a seating part of the seat of the vehicle, and a main wiring part which connects the power supply section and the main electrode. The sensor further includes a guard electrode disposed between the main electrode and the seat frame electrically connected to the ground potential level of the vehicle. The guard electrode eliminates that an electric field is formed between the seat frame and the main electrode. The sensor further includes an impedance calculation section, a real part and imaginary part calculation section, and a discriminating section. Further, in addition to the above elements, the sensor has a sub electrode and a switching section. The sub electrode is located to be apart from the main electrode, and to be next to the main electrode. The switching section switches between an occupant detection mode and a water detection mode for generating an electric field between the main electrode and the sub electrode.
In the occupant detection mode, the impedance calculation section calculates impedance through the power supply section, the main wiring part; the main electrode, and the vehicle body. The real part and imaginary part calculation section calculates a real part and an imaginary part of the impedance based on the calculated impedance. The discriminating section discriminate a passenger on the seat based on the imaginary part of the calculated impedance.
In the water detection mode, the impedance calculation section calculates the second impedance including main-sub impedance between the main electrode and the sub electrode. That is, the second impedance is impedance through the power supply section, the main wiring part, the main electrode, the sub electrode, and, the vehicle body. Hereinafter, this circuit is referred to as a water detecting circuit. In this case, a discrimination section detects water on the seat based on the imaginary part calculated in the real part and imaginary part calculation section in the water detection mode. Here, the water detection includes meaning of determination of whether water is on the seat or not, and determination of water condition, such as a discrimination between water and salted water.
In the water detection mode, it is possible to discriminate water on the seat based on the imaginary part of the second impedance of the water detecting circuit. For example, if water interposes between the main electrode and the sub electrode, the imaginary part of the main-sub impedance becomes large, and the imaginary part of the second impedance also becomes large simultaneously. Here, the imaginary part of the second impedance of the water detecting circuit corresponds to a capacitive component between the main electrode and the sub electrode, i.e., the main-sub impedance. Therefore, the discriminating accuracy for water on the seat can be improved.